1. Field of the Invention
This invention relates to scanning probe microscopes used for imaging at the interface between a fluid and a solid surface and more particularly to a microscope in which probes for both scanning tunneling microscopy (STM) and atomic force microscopy (AFM) are simultaneously scanned over a sample attached to a moveable platen or sample stage suspended or located below the scanner. This arrangement permits the use of separate probes for AFM and STIVI and yet, through relative translation of the sample, permits the same part of the sample to be examined in turn by each probe. In this arrangement, each probe may be optimized for its function (being STM or AFM).
2. The Prior Art
There are many advantages to being able to perform both STM and AFM on the same sample, particularly in the controlled conditions that may be obtained by operating under a fluid with the sample under potential control as taught in U.S. Pat. Nos. 4,868,396 and 5,155,361 to S. Lindsay. The AFM senses Van der Waals forces in the vicinity of a sample surface and thus it provides information about the local topography of a surface, or, if the surface yields under the probe, about its local stiffness. The STM provides information about the electronic properties of a surface. It is essentially a current sensing device for sensing a tunneling current in the vicinity of a sample surface. If these are homogeneous over a surface, as, for example, when the sample is a uniform metal surface, then the STM image may be interpreted in terms of local topography as well. The advantage of combining both measurements in one device is that local electronic features may be separated from local topographic features. For example, a point at which the electronic conductance is high might appear as a "high" point in an STM image. It could not be easily distinguished from a high point on a surface of uniform conductivity. However, if an AFM image is first acquired, so that the topographically high and low points in a given region may be mapped, subsequent STM images can be interpreted in terms of local conductivity. This is particularly important when these microscopes are used in the development and testing of electronic devices or other electrically non-uniform surfaces such as biological and organic molecules as well as cell membranes and the like.
Specht et al. [Surface Science letters 257, L653, 1991] have described a microscope which achieves the goal of carrying out both STM and AFM on the same region of the sample. This prior art is shown in highly schematic form in FIG. 1. The force sensing cantilever 10 is coated with a thin conducting film 12 which covers the force sensing probe 14 at the end of the cantilever 10. The probe 14 is held in contact with the sample surface 16. Deflections of the probe 14 as it is scanned over the sample surface 16 are detected by reflection of a probing laser beam 18 from the back 20 of the cantilever 10. A map of these deflections, recorded as a function of the position of the probe, constitutes the AFM image of the surface topography. The conducting film 12 may be used to record the sample conductivity if a voltage from a voltage source 22 is applied between it and the sample surface 16, and the consequent current flow across node 24 recorded. This prior art device, while fit for its intended use, has the disadvantage that it will not operate in a conducting fluid such as an aqueous electrolyte.
In order to operate an STM under fluids, it is necessary to insulate the probe so that only the very tip is exposed, as described by Sonnenfeld and Hansma [Science, 232, 211, 1986] and Nagahara et al. [Review of Scientific Instruments, 60, 3128, 1989]. This is difficult to do with a force-sensing probe. Furthermore, coating microfabricated cantilevers may impair their sensitivity and linearity as a force probe.